Well this thing has came to my head now, and can't find any info. I would like to know if it's possible to create custom HTTP errors, no custom pages, like 404 pages, I mean error codes.
The idea came when I saw Slackware's Connie server although that may be a simple index page. Well that was my question, thanks :)

Well, that page is just returning an error page with a 200 OK response, so it's nothing exciting. But there's nothing stopping you from returning a customer error code if you want, although it's not a particularly good idea unless you're only ever going to be returning it to your own custom clients that know what it means and how to deal with it. I also wouldn't be violating the standard conventions (2xx for success, 4xx for per-request errors, 5xx for server-side problems, etc)